Thank you for your enquiry.
I passed it onto our Conservation staff who made the following comment:
“I would say the blue is copper/sulphur corrosion products. The collar and hat Rising Sun hat badges were typically patinated using sulphur compounds to give the dark brown colour. In the right environmental conditions the brown copper sulphides can change to bright blue copper sulphides or sulphates.”
Additionally, we do not note any raised enclosures around the apparent “enamelled” section. Enamelled uses heated glass powder which flows, when super-heated, into the design and is corralled by a raised border to stop it escaping into areas not to be enamelled. It would appear too, that, once cleaned, this blue has disappeared – this would not happen with enamelling.
Overall, then, we suggest you have standard collar badges which were chemically darkened during manufacturing (most badges designed for operational use were darkened), and that this process, combined with time and climate, has been proved responsible for the blue copper sulphate colour evident on the badges.
I hope this is of assistance
Military Heraldry & Technology